gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
101210-1718 EST
Mike32186:
I would like present some background to help you understand your problem.
Transformer losses are primarily from two sources, core losses and I^2*R losses. The core losses are from hysteresis and eddy currents in the core. The resistance losses are in the primary and secondary wires. Note: eddy current losses are a resistive loss, but within the core and are therefore part of core losses and do not change much with load current.
All the transformer losses cause heating of the transformer, and internal temperature of a transformer determines its lifetime. On average the lower the temperature the longer the life, at least until the temperature is too low.
You can assume core losses are a constant.
If a transformer of a specific physical size and design has a KVA rating of X, then for the most part the output voltage of the secondary has little effect upon the KVA rating. Suppose the transformer had a KVA rating of 1000 VA, then if the secondary voltage rating was 100 V, the rated current for the transformer would be 10 A. For a 10 V output the current would be 100 A. Either one of these would produce about the same internal heating at full load, and nominal voltage.
So current in the secondary is your concern relative to the transformer rating.
In your Y connected secondary the secondary common to the two loads has the greatest current flow and becomes the limiting factor. This secondary will get hottest. Now your problem is to determine the current in this leg. That has been done for you above. Also you have been told that the KVA rating of a 3 phase transformer has to be divided by 3 to get the KVA rating of one secondary. The KVA rating of one secondary has to be compared with the KVA load that will be placed on that secondary.
In this application because it is a Y secondary you use 120 V with the transformer 1/3 KVA value to determine the capability of the secondary.
.
Mike32186:
I would like present some background to help you understand your problem.
Transformer losses are primarily from two sources, core losses and I^2*R losses. The core losses are from hysteresis and eddy currents in the core. The resistance losses are in the primary and secondary wires. Note: eddy current losses are a resistive loss, but within the core and are therefore part of core losses and do not change much with load current.
All the transformer losses cause heating of the transformer, and internal temperature of a transformer determines its lifetime. On average the lower the temperature the longer the life, at least until the temperature is too low.
You can assume core losses are a constant.
If a transformer of a specific physical size and design has a KVA rating of X, then for the most part the output voltage of the secondary has little effect upon the KVA rating. Suppose the transformer had a KVA rating of 1000 VA, then if the secondary voltage rating was 100 V, the rated current for the transformer would be 10 A. For a 10 V output the current would be 100 A. Either one of these would produce about the same internal heating at full load, and nominal voltage.
So current in the secondary is your concern relative to the transformer rating.
In your Y connected secondary the secondary common to the two loads has the greatest current flow and becomes the limiting factor. This secondary will get hottest. Now your problem is to determine the current in this leg. That has been done for you above. Also you have been told that the KVA rating of a 3 phase transformer has to be divided by 3 to get the KVA rating of one secondary. The KVA rating of one secondary has to be compared with the KVA load that will be placed on that secondary.
In this application because it is a Y secondary you use 120 V with the transformer 1/3 KVA value to determine the capability of the secondary.
.